Fix Use After Free for large message sizes

The buffer to receive messages is initialised to 16k. If a message is
received that is larger than that then the buffer is "realloc'd". This can
cause the location of the underlying buffer to change. Anything that is
referring to the old location will be referring to free'd data. In the
recent commit c1ef7c97 (master) and 4b390b6c (1.1.0) the point in the code
where the message buffer is grown was changed. However s->init_msg was not
updated to point at the new location.

CVE-2016-6309
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

ssl/statem/statem.c

@@ -445,6 +445,21 @@ static void init_read_state_machine(SSL *s)
     st->read_state = READ_STATE_HEADER;
 }
 
+static int grow_init_buf(SSL *s, size_t size) {
+
+    size_t msg_offset = (char *)s->init_msg - s->init_buf->data;
+
+    if (!BUF_MEM_grow_clean(s->init_buf, (int)size))
+        return 0;
+
+    if (size < msg_offset)
+        return 0;
+
+    s->init_msg = s->init_buf->data + msg_offset;
+
+    return 1;
+}
+
 /*
  * This function implements the sub-state machine when the message flow is in
  * MSG_FLOW_READING. The valid sub-states and transitions are:
@@ -545,9 +560,8 @@ static SUB_STATE_RETURN read_state_machine(SSL *s)
             /* dtls_get_message already did this */
             if (!SSL_IS_DTLS(s)
                     && s->s3->tmp.message_size > 0
-                    && !BUF_MEM_grow_clean(s->init_buf,
-                                           (int)s->s3->tmp.message_size
-                                           + SSL3_HM_HEADER_LENGTH)) {
+                    && !grow_init_buf(s, s->s3->tmp.message_size
+                                         + SSL3_HM_HEADER_LENGTH)) {
                 ssl3_send_alert(s, SSL3_AL_FATAL, SSL_AD_INTERNAL_ERROR);
                 SSLerr(SSL_F_READ_STATE_MACHINE, ERR_R_BUF_LIB);
                 return SUB_STATE_ERROR;